1. Field of the Invention
The present invention relates to an image reading device employed in an image scanner, a copier or the like for reading an image data of an original document and outputting a density data and a color discriminating data for each picture element.
2. Description of the Related Art
A conventional image reading device for reading an original image with at least two color characteristics and outputting a density data indicating the density and a color discriminating data indicating the color of each picture element has been disclosed in U.S. Pat. No. 4,553,160.
In this type of image reading device, color of each picture element is discriminated on the basis of a density data concerning all the color components (overall density data) and a density data concerning a specified color, for example, red (red density data). The overall density is detected by using, for example, a CCD sensor, while the red density is detected by using a red light transmitting filter and another CCD sensor having the same detecting element density as the above CCD sensor.
More specifically, as shown in FIG. 1, the overall density data and the red density data having the same number of bits (the same number of density level) as the overall density data are inputted as an address into a MAP-ROM 92 of a color discriminating section 91. A data corresponding to the address is read as a color discriminating data out of a color discriminating map which is previously stored in the MAP-ROM 92.
In the above device, however, since amount of the data to be processed in a color discriminating circuit and other circuits is large, elements operable at high speeds are required. This causes the circuit construction to be complicated. In addition, where a delay element or a frame memory is used to synchronize the overall density data and red density data, the delay element or the frame memory is required to be large in memory capacity.
Further, since the overall density data and red density data have the same number of density level, the MAP-ROM 92 is required to have a large memory capacity and the number of lines between the CCD sensors and the MAP-ROM 92 is increased. Such a large number of density level of the red density data also requires the memory capacity of the delay memory to be large.
As apparent from the above, the conventional image reading device has problems of complicated construction, large size and high cost.